EP0591037B1 - Method for obtaining and treating data for monitoring the movement of fluids in a reservoir - Google Patents

Description

The present invention relates to a method of acquisition and processing of seismic data, used in the context of a repetitive monitoring of the movement of the fluids impregnating a reservoir located deep in the basement below a surface alteration zone .

The invention finds a particularly advantageous application in the field of applied geophysics where it is sought in particular to collect information on the movements of hydrocarbons and water in an underground reservoir by repeated reflection seismic methods.

Indeed, seismic reflection makes it possible to reconstruct deformations of the subsoil from the measurement of the propagation times of the seismic waves emitted up to the main interfaces. In addition, when the signal-to-noise ratio of the recordings is good, it is possible to measure the reflectivity of these interfaces, which is proportional to the amplitude of the reflected seismic waves. Therefore, it is possible to follow variations in lithology or porosity of the subsoil, and therefore to describe for example more precisely the nature of underground hydrocarbon tanks.

With regard to the detection of fluids in such a tank, it sometimes happens that under favorable conditions, that is to say a loose and porous tank containing gas or oil with low viscosity, we find on a seismic profile reflecting the presence of fluids or the position of their interfaces, causing amplitude anomalies or a change in configuration of seismic reflections. However, in most cases, this direct detection of fluids is not possible because of the small variations in reflectivity induced by the displacement of the fluids or because of a lack of resolution of the method used.

A technical solution to the aforementioned problems of simple reflection seismic is repetitive monitoring, that is to say a method of recording seismic reflection repeated at intervals of time sufficient for the movement of fluids in the tank to produce differences. detectable propagation time or amplitude of the reflected waves, between the successive reflection seismic profiles. By comparing two recordings, a relative measurement of the reflected seismic waves is carried out, the sensitivity of which is much higher than that of a single acquisition in the case of simple reflection seismic, since it makes it possible to identify displacements of the fluids without them these are detected directly.

It should however be specified that repetitive monitoring requires, whatever the method used to obtain the reflection seismic recordings, repeated use of the implementation devices, strictly repositioned, and recordings with a high signal-to-noise ratio and good vertical resolution.

Currently, known seismic reflection methods (US-A-2,900,037, GB-A-2,153,529) and implemented in the context of repetitive monitoring, are seismic methods from the surface in 2D or 3D, or well-to-well or mixed surface-well as the PS V (Vertical Seismic Profile). As a reference document on the geophysical methods used to carry out repetitive monitoring of fluids, one can consult the document "New Dimensions in Geophysics for Resevoir Monitoring, SPE formation Evaluation", June 1991, p. 141-150 (R.J. Greaves, W.B. Beydoun and B.R. Spies).

In general, all these above-mentioned seismic reflection acquisition methods aim to obtain a continuous image of the subsoil (on a surface, a line or near a well) and all use a method of addition on the cover. multiple to improve the signal to noise ratio of the recordings obtained.

In the context of repetitive monitoring, none of these aforementioned methods well known to those skilled in the art are satisfactory because they come up against problems of accessibility and seasonal variation in the properties of the surface levels of the sub-surface. ground, and in marine, with the difficulties of exact repositioning of the successive passages.

On land, carrying out seismic reflection from the surface in 2D or 3D is often difficult because it requires the installation of a large number of receivers over a large area, which poses accessibility problems on the land (viability, permittage) and immobilization or repositioning of an important acquisition device. In addition, the recordings obtained using surface seismic have an average signal-to-noise ratio because the seismic noise is significant at the surface. This seismic noise on the surface is a ground wave and an aerial wave. On the other hand, the resolution of these recordings is low because there is an absorption of high frequencies in the surface alteration zone of the basement to be studied, and filtering linked to interference between the receivers placed on the ground. In addition, the cost of implementing surface seismic is relatively high.

Well seismic methods provide recordings whose resolution and signal-to-noise ratio are generally adapted to repetitive monitoring, but the realization of these methods is difficult because of the limited accessibility of wells and their investigation remains punctual around only existing wells.

The present invention provides a new method of acquisition and processing of seismic data, suitable for repetitive monitoring by seismic reflection of fluids in an underground reservoir, which aims to overcome the drawbacks of the above-mentioned prior seismic methods.

• a) aménager en chaque point d'une grille prédéterminée en surface, un trou d'axe vertical de faible profondeur dans le sous-sol au-dessus du réservoir, et traversant la zone d'altération superficielle,
• b) positionner dans chaque trou suivant son axe vertical une pluralité de récepteurs fixes et destinés à être connectés séparément à un enregistreur placé en surface,
• c) émettre à proximité de chaque trou des ondes sismiques dans le sous-sol au moyen d'une source émettrice placée en surface à proximité de la verticale de ce trou,
• d) enregistrer pour chaque trou, au moyen des récepteurs placés dans ledit trou, les ondes sismiques incidentes directes et les ondes sismiques réfléchies aux interfaces des couches profondes du sous-sol, chaque récepteur fournissant un enregistrement séparé d'une onde incidente et de plusieurs ondes réfléchies,
• e) pour chaque trou, effectuer le traitement suivant :
  • pointer la première arrivée des ondes incidentes directes,
  • horizontaliser les ondes réfléchies,
  • séparer les ondes réfléchies et les ondes incidentes directes,
  • déconvoluer récepteur par récepteur les ondes réféchies par l'onde incidente directe afin d'obtenir pour chaque récepteur une trace 0-phase,
  • additionner les traces 0-phase provenant des récepteurs pour obtenir une trace 0-phase à déport nul de faible couverture.

More particularly, according to the invention, this method comprises the following steps consisting in:

• a) arrange at each point of a predetermined grid on the surface, a shallow vertical axis hole in the basement above the tank, and crossing the surface weathering zone,
• b) positioning in each hole along its vertical axis a plurality of fixed receivers intended to be connected separately to a recorder placed on the surface,
• c) emit near each hole seismic waves in the subsoil by means of an emitting source placed on the surface near the vertical of this hole,
• d) record for each hole, by means of the receivers placed in said hole, the direct incident seismic waves and the seismic waves reflected at the interfaces of the deep layers of the subsoil, each receiver providing a separate recording of an incident wave and of several reflected waves,
• e) for each hole, carry out the following treatment:
  • point the first arrival of direct incident waves,
  • horizontalize the reflected waves,
  • separate the reflected waves and the direct incident waves,
  • deconvolve receiver by receiver the waves reflected by the direct incident wave in order to obtain for each receiver a 0-phase trace,
  • add the 0-phase traces from the receivers to obtain a 0-phase trace with zero offset of low coverage.

Here we mean by a 0-phase trace, a trace made up of reflections whose maximum or minimum amplitudes are located vertically (in propagation time) of the reflective interfaces of the subsoil and whose values are proportional to the reflectivity of these interfaces. In addition, a zero offset trace is a trace equivalent to that which would have been recorded using a transmitter and a receiver placed in the same place, which corresponds to a propagation of the waves along the vertical. Furthermore, the term “low coverage trace” is understood to mean a trace formed by the addition of a reduced number of traces.

Thus, according to the method of acquisition and processing of seismic data according to the invention, at each point of the grid, a measurement corresponds to one or more seismic emissions recorded separately by each of the receivers arranged vertically in the hole, placed in this point, which extends under the surface alteration zone of the basement. Advantageously, the holes are arranged according to a grid of more or less spaced points so as to sample the reservoir in a representative manner. It will be noted in this case that the investigation is not linked to the presence and availability of deep holes.

The aim of the method according to the invention is not to reconstruct a continuous image of the basement on a given date but to obtain at each point of the grid a 0-phase trace with zero offset of low coverage, it that is to say a recording of the waves reflected vertically from the hole positioned at each point. This non-migrated trace is representative of the average reflectivity of the interfaces on the surface illuminated by the reflected waves, this illuminated surface being called the Fresnel zone.

According to the invention, for each hole, the emission of seismic waves in the basement is repeated on different dates and the 0-phase traces with low offset zero offset are compared obtained from the emissions carried out at the different dates, by correcting variations in propagation time linked to seasonal changes in the surface alteration zone on said traces, and by calculating differences in amplitude and propagation time at the level of the corresponding reflections on said traces in the sub- soil in order to obtain information on the movement of fluids in the reservoir to be studied.

According to the invention, the holes made make it possible to keep the receivers buried, available and accessible for each emission, even if the emission dates are spaced several months or several years apart. During repetitive monitoring, the method according to the invention does not require the repositioning or the immobilization of numerous receivers, as is the case for seismic methods from the surface. By repeatedly using its own vertical devices, the method according to the invention makes it possible to obtain a spatial and temporal sampling of the reservoir at intervals determined by the complexity of the reservoir, the speed of movement of the fluids and the budget available. The proposed method, which involves making shallow holes fitted with receivers, has a lower cost than the repetition of continuous profiles required by the surface sisimics of the prior art. It should be noted that according to the invention the use of buried receivers and arranged vertically makes it possible to obtain seismic traces having both a high signal to noise ratio and good vertical resolution, in particular thanks to the vertical filtering of the effects of surface and positioning of the device under the surface alteration zone of the basement.

In addition, it will be noted that according to the invention, the processing of seismic acquisitions comprises a step of level by level deconvolution of the waves reflected by a direct wave, making it possible to obtain reflections whose amplitude is representative of the reflectivity of the sub- ground vertical to the hole. This advantageous and original stage of treatment runs counter to all the assumptions of those skilled in the art. Indeed, because the receivers are shallow, the reflected waves travel a much longer path than direct waves, and the skilled person can think a priori that the operation of deconvolution of one by the other will not give a satisfactory result. In fact, the result of the deconvolution is satisfactory because the direct and reflected waves are essentially shaped by their passage through the surface alteration zone.

• La figure 1 est un schéma en perspective d'une tranche de terrain avec arrachement partiel, montrant l'installation de mise en oeuvre de la méthode selon l'invention.
• La figure 2 est une vue de détail d'un trou comportant les récepteurs permettant les enregistrements des ondes directes et réfléchies suivant la méthode d'acquisition et de traitement de données sismiques selon l'invention.
• La figure 3 est un diagramme de visualisation des ondes sismiques directe et réfléchie enregistrées par chacun des récepteurs de la figure 2.
• La figure 4 est la visualisation de la trace 0-phase obtenue après traitement pour chacun des récepteurs de la figure 2.
• la figure 5 montre la trace 0-phase à déport nul de faible couverture obtenue après addition des traces 0-phase obtenues à la figure 4.
• La figure 6 montre la correspondance entre cette trace 0-phase à déport nul, de faible couverture obtenue après traitement pour le trou de la figure 2 et les valeurs de la réflectivité des interfaces du sous-sol à la verticale dudit trou.
• La figure 7 est un exemple d'une répartition différente des fluides dans le réservoir de la figure 1 à deux dates différentes.
• La figure 8 représente deux traces sismiques enregistrées à la verticale d'un trou de la grille de la figure 1 pour les deux dates de la figure 7, et traitées selon l'invention.
• La figure 9 représente un modèle des variations des grandeurs caractéristiques de la réflectivité du réservoir de la figure 7 en fonction de la saturation en eau dans le réservoir.

The description which follows with reference to the appended drawings, given by way of nonlimiting examples, will make it clear what the invention consists of and how it can be implemented.

• Figure 1 is a perspective diagram of a section of land with partial cutaway, showing the installation for implementing the method according to the invention.
• FIG. 2 is a detailed view of a hole comprising the receivers allowing the recording of direct and reflected waves according to the method of acquisition and processing of seismic data according to the invention.
• FIG. 3 is a diagram showing the direct and reflected seismic waves recorded by each of the receivers in FIG. 2.
• FIG. 4 is the visualization of the 0-phase trace obtained after processing for each of the receptors in FIG. 2.
• FIG. 5 shows the 0-phase trace with zero offset of low coverage obtained after addition of the 0-phase traces obtained in FIG. 4.
• FIG. 6 shows the correspondence between this 0-phase trace with zero offset, of low coverage obtained after treatment for the hole in FIG. 2 and the values of the reflectivity of the interfaces of the subsoil vertical to said hole.
• Figure 7 is an example of a different distribution of fluids in the tank of Figure 1 on two different dates.
• FIG. 8 represents two seismic traces recorded vertically from a hole in the grid of FIG. 1 for the two dates in FIG. 7, and treated according to the invention.
• FIG. 9 represents a model of the variations of the quantities characteristic of the reflectivity of the reservoir of FIG. 7 as a function of the water saturation in the reservoir.

In Figure 1, an example of a portion of land is shown, with a partial view of the basement. Starting from the surface of the ground towards the deep layers of the subsoil, there is first of all a surface alteration zone 2 which extends in depth over approximately 70 meters and a reservoir 1 located approximately 1200 meters deep , containing fluids not shown in this figure, such as for example water and gas. This reservoir 1, seen in section, extends over the entire length of the portion of land shown. Furthermore, in FIG. 1, the implementation of a recording is distinguished from the method of acquisition and processing of seismic data according to the invention, aiming in particular to follow the displacements of the fluids in the reservoir 1.

As regards the acquisition of seismic data, according to this method, as can be seen in FIG. 1, there is provided at each point A with a predetermined grid on the surface, a hole 100 of vertical axis UU 'and of shallow depth in the basement above the reservoir 1. The points A of the grid are more or less spaced from each other, here for example from 250 to 500 meters, so as to correctly sample the reservoir 1. Here, the grid has about twenty points A. Each hole 100 of the grid extends from the surface to a depth here of about 150 meters, inside an environment in which seismic waves propagate preferably at constant speed and which is located under the surface alteration zone 2. These holes 100, not cased or comprising a simple polyvinyl chloride jacket for example, are made with the conventional drills used for the preparation of VT core samples, necess areas to measure the propagation times in the superficial levels. Each hole 100 has a diameter of about 10 centimeters.

As can be seen in Figures 1 and 2, in each hole 100, is positioned along its vertical axis U-U 'at least six to eight receivers 101,102,103,104,105,106,107,108, which are connected separately to the surface. These receivers are distant in the hole from two to ten meters approximately depending on the speed of the waves emitted in the environment surrounding the receivers and the frequency of the waves emitted. The receivers are lowered and cemented directly in each hole 100, so that they are fixed in said hole. Each receiver is provided with a box reinforcing its tightness and is connected to the surface to one or more waterproof sockets located under a manhole 110 placed on the surface. When a measurement is made on a hole, as will be explained later, the receivers of the hole in question are then connected separately to a recorder 300 which is connected to the waterproof socket (s) located under the manhole 110. The receivers used are for example geophones or triphones. For reasons of space, the triphones can consist of three separate mini-sensors, arranged in a plastic tube. Their orientation in the hole can be determined relative to the direction of a transmission carried out at the surface with a certain offset.

As shown in FIGS. 1 and 2, in a following step of the method according to the invention, for each hole 100, seismic waves are emitted in the subsoil by means of an emitting source 200 placed on the surface near to the vertical UU 'of each hole 100. Here the emitting source 200 is placed about 20 meters from the vertical of the hole in question. Preferably, several transmissions are made at each recording, either on site to improve the signal to noise ratio of the recordings, or by gradually shifting so as to obtain a collection of traces around the vertical UU 'of the hole 100. However, the low The depth of the receptors can limit the effectiveness of the proposed treatment to offsets not exceeding the average depth of these receptors, in this case around 100 meters on either side of the hole 100.

As shown in FIGS. 2 and 3, each emission is at the origin on each of the receivers of the hole 100 recorded separately, of a direct arrival of direct incident wave or downward wave 10, and of arrivals of waves reflected at the interfaces of the basement, formed in particular by the fluids of the reservoir 1, or rising waves 20. The waves are recorded and transmitted to the recorder 300.

Thus, each emission or group of emissions on each hole provides one or more seismic recordings on a given date which will make it possible to measure the reflectivity of the subsoil vertically from each hole. The record or records obtained on a given date are transformed for each hole according to the processing steps of the method according to the invention. It should be noted that in the context of repetitive monitoring, the dates for taking seismic measurements or recordings can be spaced several months or even several years apart.

We will now describe the processing steps of the method according to the invention for an emission near a hole, these being shown more particularly in FIGS. 4, 5 and 6.

For each hole, the first arrivals of the direct incident waves 10 are pointed so as to measure the propagation time of these waves, to the various receivers. These delays are then used in two ways: they are added to the arrival times of the various reflected waves 20, which has the effect of horizontalizing them; they are used to identify and correct seasonal variations in speed in the surface alteration zone. The rising (reflected) waves 20 and the falling (direct) waves 10 are then separated by filtering in frequency and in number of waves knowing that said rising and falling waves do not have the same apparent speed. Then, the receiver or receiver deconvolves the rising or reflected waves 20 by the falling or direct wave 10 arriving at the receiver in question in order to obtain for each receiver a 0-phase trace 30, as shown in FIG. 4. These traces 0 -phase present reflections whose times of propagation and the maximum or minimum amplitudes are proportional to the depth and the reflectivity of the different interfaces of the subsoil and are independent of the surface conditions after correction of the variations. It should be noted that the success of this deconvolution supposes that the rising 20 and falling 10 waves are almost entirely modeled during their passage through the surface alteration zone. This is the condition for the downward wave 10 to be representative of the upgoing waves 20, the path of which under said alteration zone 2 is much longer than that of the downgoing waves 10, and therefore that the deconvolution is effective. In the example proposed, the down waves travel around 100 meters while the up waves travel around 2000 to 2500 meters. In the PSV method, where the receptors are lowered into the well as close to the reservoir, this difference in path is much less.

As a result, after adding the horizontalized 0-phase traces, a 0-phase trace with zero offset of low coverage 40 is obtained, as shown in FIG. 5, that is to say that for this new trace 40 everything is happening as if the propagation of the waves had taken place vertical to the said hole. FIG. 6 shows the correspondence between this 0-phase trace with zero offset 40, and the reflectivity R ₁ , R ₂ of the deep interfaces of the subsoil around the hole 100 considered. It should be noted that according to the acquisition and processing method described here, it is possible at the same time to compensate for the seasonal changes in the propagation times in the surface alteration zone by measuring the delays on the down waves 10 ( pointed at first arrivals), and take into account the amplitude variations of the emission thanks to the deconvolution, that is to say that we can ignore all the superficial variations from one acquisition to another .

According to the embodiment described in Figures 1, 2, 3, 4, 5 and 6, the method of acquisition and processing of seismic data allows, on the one hand, on a given date to obtain a sampling of the reflectivity of basement according to the grid shown in Figure 1. On the other hand, it allows by repeating on dates spaced in time J ₁ and J ₂ the seismic emissions near the different holes 100 of the grid, and comparing for each hole the different 0-phase traces with zero offset obtained at the end of the treatment, to obtain an evaluation of the variations in saturation of the fluids in the reservoir 1 distributed in a discrete manner.

For example, as shown in FIG. 7, it is assumed that the reservoir 1 comprises two fluids 1a, 1b, such as gas and water separated by a gradual transition, which between dates D ₁ and D ₂ will move towards the top as a result of gas production. On date J ₁ and on a vertical U-U ', the reservoir I has a hydrocarbon saturation of approximately 60%, and a water saturation of 40%, while subsequently, on date D ₂ , the reservoir I has, along the same vertical line, reversed hydrocarbon / water saturations as a result of the rise in the transition zone. By applying the method described in Figures 1 to 5, on dates D ₁ and D ₂ and more particularly by carrying out one or more transmissions near each hole in the grid, on dates D ₁ and D ₂ , by processing the records obtained and by comparing for each hole the traces with zero offset, as we described above, we obtain for this hole the diagram of FIG. 8 which shows, on the one hand a variation ΔA of the maximum or minimum amplitudes proportional to the variation ΔR of the reflectivity of the interfaces of the deep layers to the vertical of the hole (therefore with the variation ΔZ of acoustic impedance) and on the other hand, a variation of propagation time Δt until the interface considered which translates a variation of seismic velocity in the layers traversed by the reflected waves 20.

From the use of theoretical diagrams of the type represented in FIG. 9, it is possible to relate the variations in impedance ΔZ or speed V, measured from Δ A and Δ t, to variations in fluid saturation. and therefore to their movements.

The method of acquisition and processing of seismic data described above can be carried out in the marine environment. Each point of the grid would then correspond to a vertical flute comprising several hydrophones or groups of hydrophones, immersed above the bottom and connected to an autonomous recorder located in a sealed sphere. The transmissions are made directly above the flute from a seismic vessel which collects the plotter and recorder unit after each acquisition.

Claims (2)

1. Method of acquiring and processing seismic data for repetitive monitoring of displacement of fluids impregnating a reservoir (1) deep in the sub-surface below a surface weathered zone (2), characterised in that it comprises the steps of:

   a) making at each point (A) of a predetermined grid on the surface a vertical axis (U-U') shallow borehole (100) in the sub-surface above the reservoir (1) passing through the surface weathered zone (2),

   b) positioning in each borehole (100) along its vertical axis (U-U') a plurality of fixed receivers (101, 102, 103, 104, 105, 106, 107, 108) adapted to be connected separately to a recorder (300) on the surface,

   c) emitting near each borehole seismic waves into the sub-surface by means of an emitter (200) on the surface near the vertical axis (U-U') of said borehole (100),

   d) recording for each borehole by means of the receivers (101, 102, 103, 104, 105, 106, 107, 108) placed in said borehole (100) the direct incident seismic waves (10) and the seismic waves (20) reflected at the interfaces of deep strata of the sub-surface, each receiver providing a separate record of an incident wave (10) and a plurality of reflected waves (20),

   e) carrying out the following processing for each borehole (100):

   - picking the first break of direct incident waves (10),

   - horizontalising the reflected waves (20),

   - separating the reflected waves (20) and the direct incident waves (10),

   - deconvoluting receiver by receiver the reflected waves (20) by the direct incident wave (10) in order to obtain a 0-phase trace for each receiver,

   - stacking the 0-phase traces from the receivers to obtain a low coverage zero offset 0-phase trace.
2. Method according to claim 1 characterised in that the emission of seismic waves into the sub-surface is repeated at each borehole at different dates (J₁, J₂) and said low coverage zero offset 0-phase traces obtained from emissions at the different dates (J₁, J₂) are compared, correcting any variations in travel time in said traces related to seasonal changes in the surface weathered zone and calculating amplitude and travel time differences at respective reflections on said traces in the sub-surface to obtain information on the displacement of the fluids in the reservoir under study.

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